China plans to send the Chang'e-4
lunar probe to land in the south pole region of the far side of the
moon in 2018, according to CNSA (China National Space Administration).
This lunar exploration mission will incorporate an orbiter, a robotic
lander and rover. Chang'e-4 will be China's second lunar lander and
rover. The Chang'e-4 lander was a backup to the Chang'e-3 mission, so
it will have the same basic structure, but a different scientific
payload, holding 11 instruments. 1)2)

With its special environment and
complex geological history, the far side of the moon is a hot spot for
scientific and space exploration. However, landing and roving there
requires the relay satellite to transmit signals. Hence, the project
plans to send a relay satellite for Chang'e-4 to the halo orbit of the
Earth-Moon Lagrange Point L2 in late May or early June 2018, and then
launch the Chang'e-4 lunar lander and rover to the Aitken Basin of the
south pole region about half a year later, said Tongjie Liu, deputy
director of the CNSA's Lunar Exploration and Space Program Center. 3)4)

The lander of Chang'e-4 will be
equipped with descent and terrain cameras, and the rover will be
equipped with a panoramic camera. Like China's first lunar rover Yutu,
or Jade Rabbit, carried by Chang'e-3, the rover of Chang'e-4 will carry
subsurface penetrating radar to detect the near surface structure of
the moon, and an infrared spectrometer to analyze the chemical
composition of lunar samples.

But unlike Chang'e-3, the new
lander will be equipped with an important scientific payload especially
designed for the far side of the moon: a low-frequency radio
spectrometer.

"Since the far side of the moon is
shielded from electromagnetic interference from the Earth, it's an
ideal place to research the space environment and solar bursts, and the
probe can 'listen' to the deeper reaches of the cosmos," Liu said.

The Chang'e-4 probe will also carry
three scientific payloads, respectively, developed by the Netherlands,
Sweden and Germany, according to Liu.

The low-frequency radio
spectrometer, developed in the Netherlands, will be installed on the
Chang'e-4 relay satellite. The Dutch and Chinese low-frequency radio
instruments will conduct unique scientific studies such as measuring
auroral radio emissions from the large planets in the solar system,
determining the radio background spectrum at the Earth-Moon L2 points,
creating a new low-frequency map of the radio sky, and detecting bright
pulsars and other radio transient phenomena.

"The Chinese and Dutch
low-frequency radio spectrometers on the lander and relay satellite of
Chang'e-4 might help us detect the 21 cm hydrogen line radiation and
study how the earliest stars were ignited and how our cosmos emerged
from darkness after the Big Bang," said Xuelei Chen, an astronomer with
the National Astronomical Observatories of the Chinese Academy of
Sciences.

The rover will also carry an
advanced small analyzer for neutrals, developed in Sweden, to study the
interaction between solar winds and the moon surface.

And a neutron dosimeter, developed
in Germany, will be installed on the lander to measure radiation at the
landing site. Scientists say it is essential to investigate the
radiation environment on the lunar surface, in preparation for human
missions to the moon.

Some background:

The Chang'e-4 mission will
also be the first time that a satellite is sent to an unexplored region
on the far side of the Moon. This region is none other than the South Pole-Aitken Basin,
a vast impact region in the southern hemisphere. Measuring roughly
2,500 km in diameter and 13 km deep, it is the single-largest impact
basin on the Moon and one of the largest in the Solar System. 5)

This basin is also source of great
interest to scientists, and not just because of its size. In recent
years, it has been discovered that the region also contains vast
amounts of water ice. These are thought to be the results of impacts by
meteors and asteroids which left water ice that survived because of how
the region is permanently shadowed. Without direct sunlight, water ice
in these craters has not been subject to sublimation and chemical
dissociation.

Since the 1960s, several missions have explored this region from orbit, including the Apollo 15, 16 and 17 missions, the LRO (Lunar Reconnaissance Orbiter) and India’s Chandrayaan-1
orbiter. This last mission (which was launched in 2008) also involved
sending the Moon Impact Probe to the surface to trigger the release of
material, which was then analyzed by the orbiter.

The significance of soft landing exploration on the Lunar farside (Ref. 9).

At Lunar farside, numerous highland
terrains are distributed all over, including the highest peak, which is
up to 10.9 km. In the highland area, craters and mountains are widely
spread. The well-known South-Pole Aitken Basin (SPA) is located in the
southern part of the farside, with the central area at latitude
40°–60°S, longitude around 180° and the diameter
ranging from 2000–600 km. This is the largest-scaled and eldest
impacted basin in the solar system and of high scientific interest. 6)

The farside of the Moon, for its
special location, is of unique peculiarity that the nearside cannot
match. On one hand, the farside shields all kinds of radio waves
emitted from the Earth, thus becomes the best place for cosmic radio
spectrum detection. On the other hand, the original information of the
Moon is hidden in the largest, deepest and eldest SPA. It is crucial
for the study of the history, evolution, composition and components of
the deep-layer of both the Moon and the Earth system. Besides, how SPA
is formed remains controversial and deserves further research. Soft
landing on the SPA as well as rovering exploration of it are of great
scientific significance mainly in the following two aspects.

Planetary formation and evolution:

1) The study of SPA may benefit the
discovery of material composition of Lunar crust and mantle. So it
opens an important window to the study of the deep-layer material
composition of the Moon.

2) SPA is a basin (its altitude is
13 km lower than its surrounding highlands) and of thin crust. Whether
in the passive or active modes that bring out the Lunar mare basalt,
there should have emerged large amount of basalt in SPA. However,
currently obtained data cannot effectively prove that the basin has
abundant basalt. On the other hand, absence of basalt may indicate
something happened in the process of Lunar thermal evolution and
differentiation in early times.

3) Comparing the craters in SPA with
the Lunar mare we can see that the degradation situation of SPA is not
obvious. Also no crater with Lunar rays has been discovered. Therefore
the formation, evolution, topography and chemical characteristics of
craters in the SPA are apparently different from those of other
terrains.

Figure 1:
Elevation diagram of the South-Pole Aitken Basin (scale in km). This
LOLA (Lunar Orbiter Laser Altimeter) image on NASA's LRO (Lunar
Reconnaissance Orbiter) mission centers on the SPA (South Pole-Aitken)
basin, the largest impact basin on the Moon (diameter = 2600 km), and
one of the largest impact basins in the Solar System. The distance from
its depths to the tops of the highest surrounding peaks is over 15 km,
almost twice the height of Mount Everest on Earth. SPA is interesting
for a number of reasons. To begin with, large impact events can remove
surficial materials from local areas and bring material from beneath
the impact craters to, or closer to, the surface. The larger the
crater, the deeper the material that can be exposed. As SPA is the
deepest impact basin on the Moon, more than 8 km deep, the deepest
lunar crustal materials should be exposed here. In fact, the Moon's
lower crust may be revealed in areas within SPA: something not found
anywhere else on the Moon (image credit: NASA/GSFC)

The astronomical observation of
radio waves is one of the most effective methods to study and
understand the universe. At present, most portion of the spectrum has
been detected, such as ultraviolet wave (in 1890s), radio wave
(wavelength less than meters, in 1930s), X-ray (in 1940s), infrared and
millimeter wave (in 1950s), Gamma-ray (in 1960s). But no myriametric
wave (<30 MHz) has been detected yet. The detection of myriametric
wave is of much importance for all-sky imaging obtained by continuous
sky scanning of discrete radio source, cosmic dark times study (21 cm
radiation in dark times), solar physics, space weather,
extreme-high-energy cosmic ray and neutrino study. 7)

Interfered by ionosphere and Earth
radio waves, it is impossible to detect myriametric wave on the Earth.
In earlier times, wave detection satellites are RAE-A/B (NASA). RAE-A
was launched in 1968 and operated in near-Earth orbit. Its scientific
objective was to detect the intensity of cosmic ray (0.2–20 MHz).
But it was interfered by radio waves in Earth orbit. RAE-B was launched
in 1973 and was injected into the lunar orbit, whose scientific
objective was to detect the long-wavelength radio waves (working
frequency 25 kHz–13.1 MHz). It demonstrated that the lunar
farside is ideal for myriametric wave detection. 8)

At present, low-frequency radio
detection was mainly achieved via spacecraft operating in circumlunar
orbit by foreign countries but none of them has done this on the Lunar
surface.

The exploration of Change’4
will further promote people’s understanding of the farside of the
Moon. With comprehensive analysis and study on the nearside exploration
data, more general understanding about the Moon will be obtained and
the reliability of a theoretical system will be increased.

Engineering difficulties

To land on the farside and carry on
in-situ exploration has become one of the great targets for lunar
exploration of all nations. Nevertheless, no country had success in
landing on that side. One of the major reasons is that landing on the
farside of the moon needs to meet more technical challenges. Compared
with the nearside, the technical difficulties of landing on the farside
involve the following three aspects.

To ensure the landing safety,
guidance, navigation and control method should be optimized to adapt to
the complex terrain. On the other hand, with careful orbit design and
control, the distribution of landing sites is reduced for landing
safety.

1) Influence of the topographic
relief. In the descent process, the length of path of the lander is
almost 450 km on the Lunar surface. During this process, the distance
and velocity should be measured for navigation of the descent. At the
farside of the Moon, the topographic relief is more apparent than that
at the nearside, with the topographic elevation difference increased
from 3 km to 7 km. The difference brings out the jump of navigation
information and extreme difficulties for control strategy. Therefore,
the sectional control target and navigation information utilization
time as well as navigation algorithm should be optimized to decrease
the influence of the great topographic relief.

2) Decrease of the backscattering
coefficient. Research results show that the site on which
Change’3 has landed at the nearside of the Moon is opposite to
the SPA(South-Pole Aiken) basin at the farside of the Moon, and the
contents of FeO and TiO2 are 15%–25% and 0%–15%,
while the average value of them on the nearside of the Moon are
15%–20% more than that on the farside. The low contents of FeO
and TiO2 decrease backscattering coefficient of microwaves
and directly influence the echo characteristics of range and velocity
microwave sensor (RVS). Therefore, the signal emission energy and
signal-noise-ratio should be increased to obtain useful measuring data.

3) Reducing the distribution of
landing sites. Due to the complex terrain of the farside, it is
difficult to find out a large flat area. For different launch windows,
the landing sites may be widely distributed without precise orbit
control and descent process control. Different from Change’3, the
circumlunar orbits with various inclination angles for different launch
windows are designed for Change’4 specific requirements.
Meanwhile, the orbit is carefully regulated in the circumlunar phase.
With in-orbit calibration of the 7500 N thrust-variance engine, the
powered descent process path is precisely controlled and the landing
sites are reduced to a flat and small area.

For data collection of the
temperature of the Lunar soil in moon night, the temperature sensor has
to have a power supply. Currently, the most realistic method is to
utilize the RTG, which is based on the Seebeck principle to transform
the heat energy into the electric energy. When generating electric
power, the RTG can also supply great heat energy to regulate
temperature. Generally Pu-238 is used as the RTG source (Ref. 2).

Compared with the RTG used in
Change’3, the thermoelectric module should be increased in the
RTG of Change’4, which may mean various technical difficulties,
including high-performance thermoelectric material manufacture,
high-performance thermoelectric components design and assembly,
high-efficiency heat transformation, etc.

1) High-performance thermoelectric
material manufacture technology. Thermoelectric material is the core
material in the RTG. The electric and mechanical performance and the
thermal stability directly determine the reliability and stability of
the RTG. Currently, PbTe and CoSb3 are used, while PbTe is
well developed but with lesser performance. For further design, the
chemical dose ratio adjustment, the doping impurity selection and
manufacture process should be optimized to make the material meet the
engineering requirement.

2) High-performance thermoelectric
components design and assembly technology. The thermoelectric
components are comprised of π type pairs of the P-type and N-type
materials and directly determine key parameters such as the output
voltage, the inner resistor and the output power of the RTG. The design
and assembly of the energy transfer components determine the
reliability, including the components joint process design, the
matching design for the thermal expansion coefficient of the electrode
material and substrate material and the shelter layer and transition
layer optimization. For future improvements, the different temperatures
in both nighttime and daytime on the Moon should be considered.
Calculation, simulation, test and verification should be done to
satisfy engineering requirements.

3) High-efficiency thermoelectric
transformation technology. The thermoelectric transformation efficiency
and decay rate of the RTG directly determines the lifetime and the
performance of the RTG and is a vital segment for the success of the
mission in moon night. The key technology is thermal flow design of the
RTG, which aims to maximize the heat transformation from thermoelectric
components to heat collector. Meanwhile, the RTG shell should have the
layer of high emission ratio and low-absorption ratio to ensure the
safe of RTG under the high temperature at Lunar daytime.

For decades, this basin has been a
source of fascination for scientists; and in recent years, multiple
missions have confirmed the existence of water ice in the region.
Determining the extent of the water ice is one of the main focuses of
the rover mission component. However, the lander will also to be
equipped with an aluminum case filled with insects and plants that will
test the effects of lunar gravity on terrestrial organisms.

These studies will play a key role
in China’s long-term plans to mount crewed missions to the Moon,
and the possible construction of a lunar outpost. In recent years,
China has indicated that it may be working with ESA (European Space
Agency) to create this outpost, which ESA has described as an
“international Moon village” that will be the spiritual
successor to the ISS.

Figure 2: The Chinese lunar
rover, part of the upcoming Chang'e-4 mission to the far side of the
Moon Image credit: CASC/China Ministry of Defense)

Spacecraft:

The Chang'e-4 mission will consist
of a relay orbiter, a lander and a rover, the primary purpose of which
will be to explore the geology of the South Pole-Aitken Basin.
Chang’e-4 is the backup to the Chang’e-3 mission which put
a lander and rover on Mare Imbrium in late 2013. Following that
success, the lunar craft have been repurposed for a pioneering landing
on the moon’s far side s to communicate via the Chang'e-4 Relay
(Queqiao) satellite. 9)

Figure 3: Illustration of
China's Chang'e-4 rover in the South Pole-Aitken Basin on the far side
of the moon (image credit: CNSA, CASC, Ref. 10)

The launch mass of Chang'e-4 spacecraft is ~ 3780 kg; the lander has a mass of ~1200 kg, while the rover has a mass of 140 kg.

Development status of Chang'e-4 mission:

• November 2018: The image of Figure 4 was provided by Ref. 11) showing the preparations for the Chang'e-4 mission.

• On 16 August 2018, Chinese
scientists unveiled their Chang'e 4 lander and rover mission, saying it
will be launched in December 2018 on a mission to land on the far side
of the moon. 10)

- The announcement was made at a
news conference in Beijing, held by SASTIND (State Administration of
Science, Technology and Industry for National Defense), which oversees
China’s space activities. The configuration of the spacecraft and
a contest to name the mission rover were also unveiled.

- The Chang’e-4 spacecraft
will target a landing region within the South Pole-Aitken Basin, a vast
impact crater of immense scientific interest, with potential landing
areas previously identified in and around the Von Kármán
crater. The final landing site is understood to have been selected but
has not been revealed.

Figure 5: Artist's rendering of the Chang’e-4 lunar far side lander, released in August 2018 (image credit: CNSA, CASC)

Legend to Figure 5:
Visible on the newly released image of the lander are the antennas for
the LFS (Low Frequency Spectrometer), which will take advantage of the
uniquely quiet electromagnetic environment offered by the far side of
the moon.

- Images displayed displayed at the
press conference showed the rover was a rectangular box with two
foldable solar panels and six wheels. It is 1.5 meters long, 1 meter
wide and 1.1 meters high.

- Wu Weiren, chief engineer of
China's lunar exploration program, said the Chang'e 4 consists of two
parts - a lander and a rover, and both carry multiple scientific
instruments. The probe's design is based on its predecessor, the
Chang'e-3 (Yutu), but with some modifications.

Launch: The Chang'e-4 lander
and rover mission was launched on 7 December 2018 (18:24 UTC, local
time 02:24 on 8 December) from the XSLC (Xichang Satellite Launch
Center) in the southwest of the country atop a Long March 3B/E launch
vehicle (Ref. 10). 11)12)

Figure 6: Launch of the Long
March 3B rocket carrying Chang'e-4 on 7 December 2018 at the Xichang
Satellite Launch Center in southwest China (image credit: CASC)

CASC (China Aerospace Science and
Technology Corporation), the main contractor for China’s space
program, officially announced success of the launch following
trans-lunar injection, just under an hour after launch. The spacecraft
now enters a five-day voyage to the moon before lunar orbit injection.

No official date has been released
for the landing attempt, but CASC announced shortly after launch that
the landing will take place in the first days of January 2019,
following sunrise over the main candidate landing within the Von
Kármán crater in late December.

As the far side of the moon never
faces the Earth, communications with the spacecraft will be facilitated
by the ‘Queqiao’ relay satellite launched in May and inserted into a halo orbit
around the second Earth-moon Lagrange point in June 2018. - From this
vantage point between 65,000-85,000 kilometers beyond the moon the
Queqiao satellite will have constant line-of-sight with both the
Chang’e-4 spacecraft and Chinese ground stations in China, at
Kashi and Jiamusi, Namibia and Argentina.

Possible landing sites of Chang'e-4

The South Pole-Aitken Basin (SPA)
is large—roughly 2,500 km in diameter. A number of teams,
including some scientists from outside China, are looking at possible
landing areas around Von Kármán crater and the Apollo
basin. Phil Stooke, who produced a fascinating post on how China
decides where to land its upcoming Moon missions, has listed and mapped
the areas noted in papers on potential landing sites in the SPA, listed
S1 through S5 (Ref. 51).

Figure 7: A map of the lunar far
side showing known candidate landing areas for Chang’e-4. The red
strip represents the most likely region as of August 2018 (image
credit: Phil Stooke)

The candidate landing region is
45°S-46°S and 176.4°E-178.8°E, which is in the southern
floor of the Von Kármán crater, within the SPA basin
(Figure 8a). Von Kármán is
a pre-Nectarian crater of 180 km in diameter. Mare basalt flows filled
the crater floor subsequently at ~3.35 Ga, but a portion of possible
central peak remains near the center of the crater (Figure 8
b). NRC (National Research Council) has previously identified goals for
future sample return mission in Von Kármán crater,
including the possibility to study the existence and extent of
differentiation of the SPA melt sheet and possible exposed upper mantle
materials. 13)

The following two aspects are
carried out during the power descent phase: first, the oblique forward
motion trajectory is changed to the vertical down trajectory after the
main deceleration phase. Furthermore, the pointing of ranging is
consistent with the location of the landing point. Second, in order to
ensure the correctness of navigation results in altitude, the range
sensor is introduced to modify the navigation filtering algorithm. The
power descent of Chang'E-4 is shown in Figure 10. 14)

Figure 11: Artist's rendition of
the Chang'e-4 lander on the moon. China’s Chang’e-4 mission
on the far side of the Moon is employing an ESA-developed LEON
microprocessor (image credit: ESA)

- The ordinary computer chips you
use every day in your phone or laptop would be rapidly degraded by the
radiation and environmental extremes of space. Specialized chips are
therefore essential for spacecraft.

- Chang’e-4 touched down
inside the Von Kármán crater on the Moon’s far side
near the south pole on 3 January 2019. The lander and the rover it
delivered are currently hibernating during the lunar night, having
survived seven month-long lunar days so far.

- “Most ESA missions launched
after about 2010 include at least one LEON chip, and hundreds of these
radiation-hardened off-the-shelf chips have also been sold to space
missions both in Europe and around the globe,” explains ESA
microelectronics engineer Agustin Fernandez-Leon.

- “This number increases to
the thousands if we additionally count customizable fully programmable
gate arrays using LEON cores,” adds ESA microelectronics engineer
Roland Weigand. “The overall scale of usage is such that it is
impractical to keep track of all the missions making use of our
microprocessor technology, but it is always nice to find out.”

- Israel’s ill-fated Beresheet lander – which made it most of the way to the lunar nearside before crashing
–similarly utilized a GR712RC microprocessor, powered by two
LEON3 cores. In this instance their design was not developed by ESA
itself but privately funded by Sweden’s Cobham Gaisler company, advancing the existing LEON architecture.

• June 28, 2019: The lander
and the rover of the Chang'e-4 probe have resumed work for the seventh
lunar day on the far side of the moon after "sleeping" during the
extreme cold night. 16)

- The lander
woke up at 9:45 a.m. Thursday, and the rover, Yutu-2 (Jade Rabbit-2),
awoke at 1:26 p.m. Wednesday. Both are in normal working condition,
according to the Lunar Exploration and Space Program Center of the
China National Space Administration (CNSA).

- The rover has traveled more than 212 meters on the moon to conduct scientific exploration on the virgin territory.

- The scientific tasks of the
Chang'e-4 mission include low-frequency radio astronomical observation,
surveying the terrain and landforms, detecting the mineral composition
and shallow lunar surface structure and measuring neutron radiation and
neutral atoms.

• May 15, 2019: A lunar lander
named for the Chinese goddess of the moon may have lessened the mystery
of the far side of the moon. The fourth Chang'E probe (CE-4) was the
first mission to land on the far side of the moon, and it has collected
new evidence from the largest crater in the solar system, clarifying
how the moon may have evolved. The results were published on 15 May
2019, in Nature. 17)18)

Figure 12: An image captured by Chang'E 4 is showing the landscape near the landing site (image credit: NAOC/CNSA)

- A theory emerged in the 1970s that
in the moon's infancy, an ocean made of magma covered its surface. As
the molten ocean began to calm and cool, lighter minerals floated to
the top, while heavier components sank. The top crusted over in a sheet
of mare basalt, encasing a mantle of dense minerals, such as olivine
and pyroxene.

- As asteroids and space junk
crashed into the surface of the moon, they cracked through the crust
and kicked up pieces of the lunar mantel.

-
"Understanding the composition of the lunar mantel is critical for
testing whether a magma ocean ever existed, as postulated," said
corresponding author Chunlai Li, a professor of the National
Astronomical Observatories of Chinese Academy of Sciences (NAOC). "It
also helps advance our understanding of the thermal and magmatic
evolution of the moon."

- The evolution of the moon may
provide a window into the evolution of Earth and other terrestrial
planets, according to Li, because its surface is relatively untouched
compared to, say, the early planetary surface of Earth.

- Li and his team landed CE-4 in the
moon's South Pole-Aitken (SPA) basin, which stretches about 2,500
kilometers—about half the width of China. CE-4 collected spectral
data samples from the flat stretches of the basin, as well as from
other smaller but deeper impact craters within the basin.

- The researchers expected to find a
wealth of excavated mantle material on the flat floor of the SPA basin,
since the originating impact would have penetrated well into and past
the lunar crust. Instead, they found mere traces of olivine, the
primary component of the Earth's upper mantle.

- "The absence of abundant olivine
in the SPA interior remains a conundrum," Li said. "Could the
predictions of an olivine-rich lunar mantle be incorrect?"

- Not quite. As it turns out, more
olivine appeared in the samples from deeper impacts. One theory,
according to Li, is that the mantle consists of equal parts olivine and
pyroxene, rather than being dominated by one or the other.

- CE-4 will need to explore more to
better understand the geology of its landing site, as well as collect
much more spectral data to validate its initial findings and to fully
understand the composition of the lunar mantle.

• April 30, 2019: The lander
and the rover of the Chang'e-4 probe have resumed work for the fifth
lunar day on the far side of the moon after "sleeping" during the
extreme cold night. 19)

- The lander woke up at 7:40 a.m.
Monday (29 April), and the rover, Yutu-2 (Jade Rabbit-2), awoke at 1:45
p.m. Sunday. Both are in normal working condition, according to the
Lunar Exploration and Space Program Center of the China National Space
Administration.

- The rover has traveled an accumulated 178.9 meters on the moon and worked about one month longer than its designed life.

• April 15, 2019: The lander
and the rover of the Chang'e-4 probe switched to dormant mode for the
lunar night on Friday (12 April), with the rover traveling an
accumulated 178.9 meters on the far side of the moon. 20)

- The rover Yutu-2, or Jade
Rabbit-2, is expected to awaken again on 28 April, and the lander to
awaken the following day, according to the Lunar Exploration and Space
Program Center of the China National Space Administration.

• March 7, 2019: China's lunar
rover has conducted scientific detection on some stones on the far side
of the moon, which might help scientists find out whether they are from
outer space or native to the moon. 21)

- The rover Yutu-2, or Jade
Rabbit-2, was sent to the Von Karman Crater in the South Pole-Aitken
(SPA) Basin on the far side of the moon on Jan. 3 in the Chang'e-4
mission.

- Currently, the rover has traveled
about 127 meters on the moon, and is taking a "noon break" as the
temperature on the moon rises extremely high. It's scheduled to resume
work on March 10 and switch to its dormant mode on March 13, according
to the Lunar Exploration and Space Program Center of the China National
Space Administration.

- Scientists said the rover has
conducted scientific detection on its tracks and nearby stones. The
largest stone detected has a diameter of about 20 cm, and the rover
came as close as 1.2 meters to it.

- Experts from NSSC (National Space
Science Center) under the Chinese Academy of Sciences (CAS) said they
want to figure out the origin of the stones, whether they are
aboriginal on the moon, or meteorites from outer space. If they are
aboriginal, what is the physical process of their formation?

- As a result of the tidal locking
effect, the moon's revolution cycle is the same as its rotation cycle,
and it always faces the Earth with the same side.

- The far side of the moon was regarded as a virgin territory with unique features, and scientists expect important discoveries.

- Jinsong Ping, a researcher with
the National Astronomical Observatories under CAS, said that the rocks
on the lunar surface might be sputtered body as a result of the
meteorite impact.

- Scientists
have found evidence indicating a heavy asteroid bombardment event in
the solar system around 3.9 billion years ago. And the SPA Basin might
be an impact from that period. The exploration might offer clues as to
why the bombardment occurred, said Yongliao Zou , director of the lunar
and deep space exploration division of CAS.

- The other possibility is that the
stones are aboriginal, and contain geological information different
from the lunar dust, said Ping.

- "The rocks on the far side are
more ancient. The analysis of their substance composition might help us
better understand the evolution of the moon," said Zou.

- In addition, the study on the
tracks of the rover may reveal the information about the evolutionary
history of the lunar surface over billions of years, Ping said.

- The 135 kg lunar rover Jade
Rabbit-2 is the first ever rover to drive on the moon's far side, as
well as the lightest rover ever sent to the moon.

- From the images sent back from
Chang'e-4, scientists found the area surrounding the probe is dotted
with craters of different sizes, and it's very difficult for the rover
to drive in the region.

- The rover is capable of avoiding
some obstacles. If there are obstacles in front of it, it can stop and
plan a new route on its own. It can also climb some slopes and cross
rocky terrain, according to its designers from the China Academy of
Space Technology.

- The rover has sent back pictures about the stones on the far side of the moon.

- Experts from NSSC said it's not
easy for the rover to take the pictures since it cannot move as freely
as a human. It takes a long time to move and adjust the position and
angle of the rover.

- Scientists hope Jade Rabbit-2 will
travel farther to send more images of the unknown terrain, "listen" to
the stories recorded in the ancient lunar rocks, and find more traces
of the early history of the moon and the solar system.

- Scientists said it's just the
beginning of the scientific journey of the Jade Rabbit-2, and they
expect more interesting discoveries.

• March 1, 2019: The rover and
the lander of the Chang'e-4 probe have resumed work after "sleeping"
during their second lunar night on the far side of the moon. 22)

- The lander woke up at 7:52 a.m.
last Friday (1 March), and the rover, Yutu-2 (Jade Rabbit-2), awoke at
about 10:51 a.m. last Thursday (28 February). Both of them are in
normal condition, according to the Lunar Exploration and Space Program
Center of the China National Space Administration.

- A lunar day equals 14 days on
Earth, and a lunar night is the same length. The Chang'e-4 probe
switched to a dormant mode during the lunar night due to the lack of
solar power.

- As a result of the tidal locking
effect, the moon's revolution cycle is the same as its rotation cycle,
and it always faces Earth with the same side.

- The far side of the moon has unique features, and scientists expect Chang'e-4 could bring breakthrough findings.

- The scientific tasks of the
Chang'e-4 mission include low-frequency radio astronomical observation,
surveying the terrain and landforms, detecting the mineral composition
and shallow lunar surface structure, and measuring neutron radiation
and neutral atoms.

• February 18, 2019: Five
sites on the far side of the Moon now have official names, including
Chang'e-4's landing site. The names have significance in Chinese
culture, reflecting the background of the probe's team. 23)24)

- The IAU (International Astronomical Union) Working Group for Planetary System Nomenclature
has approved the name Statio Tianhe for the landing site where the
Chinese spacecraft Chang'e-4 touched down on 3 January this year, in
the first-ever landing on the far side of the Moon. The name Tianhe
originates from the ancient Chinese name for the Milky Way, which was
the sky river that separated Niulang and Zhinyu in the folk tale "The Cowherd and the Weaver Girl".

- Four other names for features near
the landing site have also been approved. In keeping with the theme of
the above-mentioned folk tale, three small craters that form a triangle
around the landing site have been named Zhinyu, Hegu, and Tianjin,
which correspond to characters in the tale. They are also names of
ancient Chinese constellations from the time of the Han dynasty. The fifth approved name is Mons Tai, assigned to the central peak of the crater Von Kármán, in which the landing occurred. Mons Tai is named for Mount Tai, a mountain in Shandong, China, and is about 46 km to the northwest of the Chang’e-4 landing site.

Figure 13: Looking down on the
Chang'e 4 landing site; the lander is just beyond the tip of the large
arrow, rover at tip of small arrow. The image is 850 x 850 m (image
credit: NASA LROC (Lunar Reconnaissance Orbiter Camera) M1303619844LR)

- They are also names of ancient
Chinese constellations from the time of the Han dynasty. The fifth
approved name is Mons Tai, assigned to the central peak of the crater
Von Karman, in which the landing occurred. Mons Tai is named for Mount
Tai, a mountain in Shandong, China, and is about 46 km to the northwest
of the Chang'e-4 landing site.

• February 3, 2019: An
experiment that saw the first-ever plant sprouting on the moon last
month was born in a natural disaster that devastated China's
cotton-industry almost three decades ago. 25)

- Fuguang Li was one of the Chinese
agricultural scientists whose years of hard work might one day help
lead to a base and long-term human residence on the moon.

- Li was on the team that developed
the cotton seeds carried to the moon by China's Chang'e-4 probe,
leading to the first-ever sprout on the moon.

- The seed is one of the best
varieties developed by the Institute of Cotton Research (ICR) of the
Chinese Academy of Agricultural Sciences.

- After making the first-ever soft
landing on the far side of the moon on 3 January, China's Chang'e-4
mission pioneered the first mini biosphere experiment on the moon.

- A canister on the Chang'e-4 lander
contained seeds of cotton, rapeseed, potato and Arabidopsis, as well as
fruit fly eggs and some yeast, to form a simple mini biosphere.

- Images from the probe showed that only a cotton sprout was growing.

- Although the sprout couldn't
survive the extremely cold lunar night, Li, head of ICR, believed it
could help acquire knowledge for building a base and long-term
residence on the moon.

- The cotton seeds were selected for the experiment because of their outstanding performance on Earth.

- The seeds belong to a transgenic insect-resistant cotton variety developed in China and named CCRI 41, said Li.

- China suffered its worst ever
cotton bollworm infestation in 1992. In one county, the bollworms
captured (ate) in one day over a ton.

- The disaster reduced the yield of most of the cotton fields by more than half.

- Within three years, the cotton
planting area in China fell from 100 million mu (about 6.67 million
hectares) to 60 million mu (4 million hectares).

- The market share of domestic insect-resistant cotton varieties increased from 5 percent in 1999 to 98 percent in 2012.

- The plantation of the
insect-resistant cottons not only controlled the spread of bollworm,
but also reduced the use of pesticides by 70 to 80 percent in China,
said Li.

- Based on CCRI 41, Chinese scientists have bred more than 100 new cotton varieties.

Figure 14: At 8 pm on Jan. 12,
Chang'e 4 sends back the last photo of the bio test load showing that
tender shoots have come out and the plants are growing well inside the
sealed test can. It is the first time humans conducted a biological
growth and cultivation experiment on the surface of the moon (image
credit: China Daily)

• February 1, 2019: China's Chang'e 4 probe, having made the first-ever soft landing on moon's far side, found that the temperature of the lunar surface dropped to as low as minus 190º Celsius, colder than expected.26)

- This is the
first time Chinese scientists have received first-hand data about the
temperatures on the surface of the moon during the lunar night.

- The rover and the lander of the
Chang'e-4 probe have been awakened by sunlight after a long "sleep"
during their first extremely cold night on the moon, CNSA (China
National Space Administration) announced on Thursday (31 January).

- As a result of the tidal locking
effect, the moon's revolution cycle is the same as its rotation cycle,
and the same side of the moon always faces Earth. A lunar day equals 14
days on Earth, and a lunar night is the same length. The Chang'e-4
probe switched to dormant mode during the lunar night due to a lack of
solar power.

- "According to the measurements of
Chang'e-4, the temperature of the shallow layer of the lunar soil on
the far side of the moon is lower than the data obtained by the U.S.
Apollo mission on the near side of the moon," said He Zhang, executive
director of the Chang'e-4 probe project from CAST (China Academy of
Space Technology). "That's probably due to the difference in lunar soil
composition between the two sides of the moon. We still need more
careful analysis," Zhang said.

- Temperatures vary enormously
between day and night on the moon. Previously, Chinese scientists had
no data on exactly how cold it could be.

- At the end of 2013, China launched
Chang'e-3, the country's first spacecraft to soft-land on the moon. The
scientific instruments on its lander are still operating after more
than 60 lunar nights over the past five years.

- "It was a success, but Chang'e-3
was designed according to foreign temperature data," said Zhang. The
measurement of the temperature changes between the day and night on the
moon will help scientists estimate the properties of the lunar soil,
according to Zhang.

- The rover and the lander carried a radioisotope heat source, which helped keep the probe warm during the lunar night.

- The lander was also equipped with
an isotope thermoelectric cell and dozens of temperature data
collectors to measure the temperatures on the surface of the moon
during the lunar night.

- Used for the first time in a
Chinese spacecraft, the isotope thermoelectric generation technology to
transform heat into power on Chang'e-4 is a prototype for future
deep-space exploration, said Zezhou Sun, chief designer of the
Chang'e-4 probe from CAST.

- NASA's Curiosity rover also adopts
this power technology, freeing it from the sunshine, sand and dust
restrictions that have affected its predecessors Opportunity and
Spirit, he explained.

- "It is a technology that we must
master if we want to go to the moon's polar regions or farther than
Jupiter into deep space, where solar power cannot be used as the
primary power source," he said.

• January 16, 2019: One of the
cotton seeds carried to the moon by China's Chang'e-4 probe is the
first ever to sprout on the moon, according to scientists of a mini
biosphere experiment Tuesday. 27)28)

- After making the first-ever soft
landing on the far side of the moon, China's Chang'e-4 mission
pioneered the first mini biosphere experiment on the moon.

- Professor Gengxin Xie, of
Chongqing University and chief designer of the experiment, said a
canister installed on the lander of the Chang'e-4 probe contained the
seeds of cotton, rapeseed, potato and Arabidopsis, as well as eggs of
the fruit fly and some yeast, to form a simple mini biosphere.

- Images sent by the probe showed that a cotton sprout had started to grow, though no other plants were found growing.

- The cylinder canister, made from
special aluminum alloy materials, is 198 mm tall, with a diameter of
173 mm and a weight of 2.6 kg. It also holds water, soil, air, two
small cameras and a heat control system, Xie said.

- More than 170 pictures have been taken by the cameras and sent back to Earth, according to the team.

- Why were these species chosen? Xie
said potatoes could be a major source of food for future space
travelers. The growth period of Arabidopsis, a small flowering plant
related to cabbage and mustard, is short and easy to observe. Yeast
could play a role in regulating carbon dioxide and oxygen in the mini
biosphere, and the fruit fly would be the consumer of the
photosynthesis process.

- Researchers used biological
technology to render the seeds and eggs dormant during the two months
when the probe went through the final checks in the launch center and
journey of more than 20 days through space.

- After Chang'e-4 landed on the far
side of the moon on 3 January, the ground control center instructed the
probe to water the plants to start the growing process. A tube directs
natural light on the surface of the moon into the canister to allow the
plants to grow.

- The
Chang'e-4 probe entered a "sleep mode" on Sunday (6 January) as the
first lunar night after the probe's landing fell. The temperature could
drop as low as about minus 170ºC.

- "Life in the canister would not survive the lunar night," Xie said.

- The experiment has ended. The
organisms will gradually decompose in the totally enclosed canister,
and will not affect the lunar environment, said CNSA (China National
Space Administration).

- Although astronauts have
cultivated plants on the International Space Station, and rice and
Arabidopsis were grown on China's Tiangong-2 space lab, those
experiments were conducted in low-Earth orbit, at an altitude of about
400 km. The environment on the moon, 380,000 km from Earth, is more
complex.

- "We had no such experience before.
And we could not simulate the lunar environment, such as microgravity
and cosmic radiation, on Earth," Xie said.

• January 15, 2019: China
announced Friday (11 January) that the Chang'e-4 mission, which
realized the first-ever soft-landing on the far side of the moon, was a
complete success. 29)

- With the assistance of the relay
satellite Queqiao (Magpie Bridge), the rover Yutu-2 (Jade Rabbit-2) and
the lander of the Chang'e-4 probe took photos of each other.

- The scientific instruments aboard
the probe worked well, and the images taken by the probe and detection
data have been sent back to ground control, said the China National
Space Administration (CNSA).

- At 4:47 p.m. Beijing Time on
Friday, the images of the lander and rover appeared on a large screen
at the Beijing Aerospace Control Center, showing the Chinese national
flag on both the lander and the rover with landscape dotted with
craters in the background.

- A congratulatory message sent by
the Communist Party of China (CPC) Central Committee, the State Council
and the Central Military Commission hailed the Chang'e-4 mission as a
remarkable achievement in China's space program, which marks an
important stride toward China being a strong country in space
exploration.

- "From the images sent back from
Chang'e-4, we can see the area surrounding the probe is dotted with
craters of different sizes, and it's very difficult for the rover to
drive in the region," Zezhou Sun, chief designer of the Chang'e-4
probe, said at a press conference of the State Council Information
Office.

- "We'll try to find the relatively
safe areas and make a reasonable plan for the route of the rover based
on the images taken by it," Sun said.

- The rover is capable of avoiding
some obstacles. If there are obstacles in front of it, it can stop and
plan a new route on its own. It can also climb some slopes and cross
some rocks. "We haven't found any insurmountable obstacle in the
region," Sun said.

- He said the Chang'e-4 probe has
achieved the expected landing precision. The telemetry information and
images taken by a camera on the probe showed that the spacecraft
effectively avoided obstacles during its descent.

- The probe has started its
scientific exploration, focusing on studying the terrains and
landforms, lunar environment, and substances composition, said Weiren
Wu, chief designer of China's lunar program.

- "This is the first-ever
exploration on the surface of the far side of the moon. The scientific
research will be innovative and influential both at home and abroad,"
Wu said.

• January 11, 2019: China's
Chang'e-4 probe took panoramic photos on the lunar surface after it
successfully made the first ever soft-landing on the far side of the
moon. CNSA (China National Space Administration) released the
360-degree panoramic photos taken by a camera installed on the top of
the lander (Figure 16). The picture was pieced together from 80 photos taken by a camera on the lander. 31)32)

- The images were sent back via the
relay satellite Queqiao, which was operating around the second
Lagrangian point of the earth-moon system, about 455,000 km from the
earth, where it can see both the earth and the moon's far side.

- Scientists have made a preliminary
analysis on the terrains and land form the probe's surroundings
according to the panoramic pictures.

- The Chang'e-4 probe touched down
on the Von Karman Crater at the South Pole-Aitken Basin in the morning
of 3 January 2019, and the lunar rover Yutu-2 drove onto the lunar
surface late that night. — Then the rover took a "nap" as the
solar radiation raised the temperature on the lunar surface to over
100ºC. It restarted to work on Thursday (10 January).

- According to CNSA, the rover and the relay satellite are in good condition.

Figure 16:
China's Chang'e-4 probe took panoramic photos on the lunar surface
after it successfully made the first ever soft-landing on the far side
of the moon (image credit: CNSA, Xinhua)

• January 5, 2019: Lunar rover
Yutu-2 has been driving on the far side of the moon after separating
from the lander and scientific devices on both the lander and rover are
currently gathering data, CNSA (China National Space Administration)
said late Friday (4 January). 33)

- At 17:00 local time in Beijing,
the three 5-meter antennas of the low-frequency radio spectrometer on
the lander have fully spread out, said the CNSA in a statement.

- Meanwhile, Germany's LND (Lunar
Lander Neutrons and Dosimetry) experiment was turned on for testing.
The ground control has been receiving geographic and geomorphic images
of the moon's far side.

- The Yutu-2 rover, equipped with a
data transmission link to relay via the Queqiao satellite at EML-2
(Earth-Moon Liberation point-2), completed environment perception and
route planning. It has been driving on the lunar surface on schedule
and arrived at preset location A to carry out observations.

- The LPR (Lunar Penetrating Radar)
and PCAM (Panorama Camera) the rover have been operating smoothly and
other devices will begin operation according to schedule.

- According to the CNSA, in the
following days, Yutu-2 and the lander will face the challenge of
extremely high temperatures in the lunar day. Yutu-2 will enter a
"napping" mode at an appropriate time and is expected to resume moving
next Thursday (10 January).

Figure 17:
Photo provided by CNSA on 4 January 2019 shows image of Yutu-2, China's
lunar rover, at preset location A on the surface of the far side of the
moon (image credit: Xinhua, CNSA)

• January 4, 2019: China has chosen the name "Yutu-2", or Jade Rabbit-2, for its new moon rover, the China National Space Administration (CNSA) announced on late 3 January (Thursday). 34)

- CNSA made the announcement after
China's Chang'e-4 probe, comprised of a lander and a rover, landed on
the far side of the moon earlier in the day. In Chinese folklore, Yutu
is the white pet rabbit of Chang'e, the moon goddess who lent her name
to the Chinese lunar mission.

• January 4, 2019: China's
Chang'e-4 probe has landed on the SPA (South Pole-Aitken) Basin on the
far side of the moon, regarded as a virgin territory by scientists
expecting important discoveries. 35)

- "The far side of the moon has very
unique features, and has never been explored in situ, so Chang'e-4
might bring us breakthrough findings," said Yongliao Zou, director of
the lunar and deep space exploration division of CAS (Chinese Academy
of Sciences).

- As a result of the tidal locking
effect, the moon's revolution cycle is the same as its rotation cycle.
It always faces the earth with the same side, and the far side was a
mystery before the age of spacecraft.

- About 60 years ago, the Luna 3
probe of the Soviet Union sent back the first image of the moon's far
side. And about 50 years ago, three astronauts of the United States
Apollo 8 mission became the first people to see the moon's far side
with their own eyes.

- More lunar missions showed the
moon's two sides were very different: the near side has more and
relatively flat lunar mares, while the far side is thickly dotted with
impact craters at different sizes.

- "There are great differences in
terms of substance composition, terrain and landforms, structure and
the age of rocks. For instance, about 60 percent of the near side is
covered by mare basalt, but most part of the far side is covered by
lunar highland anorthosite. Of the 22 lunar mares, 19 are located on
the near side," said Zou.

- Scientists infer that the lunar
crust on the far side is much thicker than the near side. But why is
still a mystery. Only in-situ exploration might reveal the secrets.

- Exploration of the far side might help shed light on the early history of the moon, the earth and the solar system.

- The moon and the earth shared a
similar "childhood." But the traces of the remote past on earth have
been erased by geological activity. "The moon might provide us with
some insights to the early history of earth," said Yangting Lin, a
researcher from the Institute of Geology and Geophysics of CAS.

- The SPA Basin, where the Chang'e-4
probe landed, is the largest and deepest basin in the solar system,
with a diameter of 2,500 km and a depth of more than 10 km.

- "With the Chang'e-4 probe, we can
detect information hidden deeply inside the moon. I believe there will
be surprising scientific findings," Zou said.

- "The rocks on the far side are
more ancient. The analysis of their substance composition might help us
better understand the evolution of the moon," said Zou.

- Scientists have found evidence
indicating a heavy asteroid bombardment event in the solar system
around 3.9 billion years ago. And the SPA Basin might be an impact from
that period. The exploration might offer clues as to why the
bombardment occurred, said Zou.

• January 3, 2019: The
Chang'e-4 lunar probe, the latest step in China's endeavor to explore
the silver sphere, landed at 10:26 Beijing Time on the Von Karman
crater in the South Pole-Aitken basin (177.6º east longitude and
45.5º south latitude)) and then sent back a picture of the landing
site shot by one of the monitor cameras on the probe's lander, marking
the world's first image taken on the moon's far side.36)37)38)

- The successful soft landing
formally inaugurated the world's first expedition to the far side that
never faces the Earth and is expected to fulfill scientists' long-held
aspiration to closely observe the enormous region.

- The probe conducted rapid position
adjustments when it reached to an altitude 8 to 6 km above the moon.
The descent then paused for a while at an altitude of about 100 meters
as the spacecraft needed to detect and analyze the inclination as well
as possible obstacles at its preset landing site so it could
autonomously avoid hazards.

- The lander returned the first ever
images from the surface of the far side of the morning shortly after,
with images from its descent and others cameras sent to Earth via the
Queqiao relay satellite. - Almost 12 hours later, the China Lunar
Exploration Project (CLEP) announced that the rover had descended from
the lander at 14:22 UTC. 39)

- NASA
congratulated Chinese scientists on this success, which is actually the
fourth lunar probe launched by the nation. The robotic spacecraft is
carrying instruments to analyze the unexplored region’s geology
and will conduct biological experiments. The first-ever soft landing is
a major milestone in space exploration because, unlike previous moon
missions that have landed on the Earth-facing side, this is the first
time any craft has landed on the unexplored and rugged far side of the
moon. The China Daily infosite posted that this successful landing
formally inaugurated the world’s first expedition to the far side
that never faces the Earth and is expected to fulfill scientists’
long-held aspiration to closely observe the enormous region. 40)

Figure 18:
This is the first image of the moon's far side captured by China's
Chang'e-4 probe on 3 January 2019 (image credit: CNSA)

Figure 19: A view of the surface
of Von Kármán crater from the Chang’e-4 lander
descent camera (image credit: CNSA/CLEP)

Figure 20:
Technicians celebrate after the landing of the Chang'e-4 lunar probe,
at the Beijing Aerospace Control Center (BACC) in Beijing, China on 3
January 2019 (image credit: Xinhua)

Figure 21: China’s
Chang'e-4 lunar mission (lander and rover) landed in the Von Karman
Crater, located in the Aitken Basin, in the South Pole region on the
far side of the Moon, on 3 January 2019, at 02:26 UTC (10:26 Beijing
time). The Chang’e-4 lunar mission was launched by a Long
March-3B rocket from the Xichang Satellite Launch Center, Sichuan
Province, southwest China, on 7 December 2018, at 18:23 UTC (8 December
at 02:23 local time), video credit: China Central Television
(CCTV)/China National Space Administration (CNSA) Music: Moonlight
Sonata by Beethoven courtesy of YouTube Audio Library 41)

• December 12, 2018: China's
Chang'e-4 probe decelerated and entered the lunar orbit Wednesday (12
December), completing a vital step on its way to make the first-ever
soft landing on the far side of the moon, CNSA (China National Space
Administration) announced. 42)

- After flying about 110 hours from
Earth, an engine on the probe was ignited when it was 129 km above the
surface of the moon, in line with instructions sent from a control
center in Beijing at 4:39 p.m. CST (08:39 UTC ), and then the probe
slowed and entered an elliptical lunar orbit with the perilune at about
100 km at 4:45 p.m., said CNSA.

- The probe, including a lander and
a rover, was launched by a Long March-3B carrier rocket last Saturday
(7 December 2018) from the Xichang Satellite Launch Center in southwest
China's Sichuan Province, opening a new chapter in lunar exploration.

- As the rocket was able to send the
probe into orbit precisely as planned, the control center only adjusted
the probe's orbit once on Sunday and also canceled two pre-planned
orbit trimmings before the near-moon deceleration, CNSA said.

- Next, the control center will
adjust the probe's orbit around the moon and test the communication
link between the probe and the relay satellite "Queqiao," which is
operating in the halo orbit around the second Lagrangian (L2) point of
the Earth-moon system.

- Afterwards, the control center will choose a proper time to land the probe on the far side of the moon, according to CNSA.

Sensor complement of the Chang'e-4 lander (LFS, LCAM, TCAM, LND)

Bearing in mind the features of the
farside of the Moon, scientific payloads usually include a
low-frequency radio spectrometer (LFS), an infrared spectrometer, a
panoramic camera, a lunar radar, etc. The LFS is newly developed for
the farside exploration and is implemented in the lander and the rover
for the comparison and analysis of data collected (Ref. 9).

LFS (Low Frequency Spectrometer)

Low-frequency radio frequency
spectrum analyzer for detection of low-frequency radio frequency
characteristics of the sun and the moon's low-frequency radio
environment to perform low-frequency radio astronomy observations.

The LFS is used for detecting the
low-frequency electric field of the solar storm and to study the Lunar
plasma. By detecting the low-frequency electric field from the Sun, the
planetary space and the galactic space, the information of electric
magnitude, phase, time variance, frequency spectrum, polarization and
DoA (Direction of Arrivals) are collected for analysis. With features
of variation of the spatial low-frequency electric field, the Lunar
plasma environment above the landing site will be analyzed. The LFS is
configured with a three-component decomposition active antenna to
receive electromagnetic signals from the Sun and from space. Each of
the three antenna units receives one of the three orthogonal components
of the electromagnetic signals. According to radio transmission theory,
information such as the electromagnetic intensity, the frequency
spectrum, the time variance, the polarization features and the
direction of radiation source are obtained by analyzing and processing
the exploration data.

LCAM (Landing Camera)

LCAM is used for optical imaging of the landing area during descent to investigate surface morphology and geological structure.

TCAM (Terrain Camera)

The objective of TCAM is for optical imaging of the landing area to investigate surface morphology and geological structure.

LND (Lunar Lander Neutrons and Dosimetry Experiment)

The LND effort is led by the
Christian-Albrechts-University in Kiel (CAU), Germany, with
contributions from the Institute for Aerospace Medicine of the German
Space Center, the National Space Science Center (NSSC), the National
Astronomical Observatories (NAOC) from the Chinese Academy of Sciences
(CAS), and the China Academy of Space Technology (CAST). LND is
supported by DLR (German Space Agency) through the Federal Ministry of
Economics and Technology.

The LND instrument will be accommodated on the Chang'e-4 Lander and has two major science objectives: 43)

1) dosimetry for human exploration of the Moon and

2) contribute to heliospheric science as an additional measuring point.

To achieve the first objective, LND
is designed to measure time series of dose rate and of linear energy
transfer (LET) spectra in the complex radiation field of the lunar
surface. For the second objective, LND is capable to measure particle
fluxes and their temporal variations and thus will contribute to the
understanding of particle propagation and transport in the heliosphere.
Its stack of 10 silicon solid-state detectors (SSDs) allows to measure
protons from 10-30 MeV, electrons from 60-500 keV, alpha particles from
10-20 MeV/n and heavy ions from 15-40 MeV/n. In addition, LND can
measure fast neutrons in the energy range from 1-20 MeV and, using two
Gd-sandwich detectors, measure fluxes of thermal neutrons, which are
sensitive to subsurface water and important for understanding lunar
surface mixing processes.

Instrument concept: The
zenith-pointing LND is mounted inside the payload compartment of the
Chang’E 4 lander and consists of a thermally decoupled sensor
head and an electronics box (Figure 22).
The sensor head consists of a stack of 10 SSDs (Figure 4) as well as
two Printed Circuit Boards (PCBs). One PCB is used to pre-amplify the
detector signals, the other PCB contains shaper circuits and the Analog
to Digital Converters (ADCs). These digitized signals are sent from the
sensor head to the electronics box which accumulates the data into
histograms, count rates, PHA words, etc., packetizes them and sends
them to the Instrument Control Unit (ICU). The latter serves as the
electrical and data interface with the lander.

Measuring fast neutrons:
Fast neutrons are generated by the interaction of the galactic cosmic
rays with the lunar regolith and are an important source of the
radiation dose reaching the interior of an astronaut’s body. LND
uses three segmented Si SSDs which are as closely packed as possible to
detect fast neutrons. The innermost segment of the C detector in Figure
23, C1, measures neutral radiation in
anti-coincidence with all outer segments (B1&B2, D1&D2, C2).
LND’s response to neutral radiation (n, γ) is shown in
Figs. 7 and 8.

Figure 23: Measurement of fast neutrons (image credit: LND Team)

Sensor Head Design: LND is
largely based on developments which were made for IRAS (Ionizing
Radiation Sensor) in an early phase of ExoMars. As shown in Fig. 4, LND
is basically a telescope consisting of ten segmented SSDs (A-J). Three
detectors (B, C, and D) are packed as close together as possible to
measure neutrons in the energy range 1-20 MeV (see section 3). The
lower six detectors (E-J) are mounted in two different sandwich
configurations. In one each sandwich clamps a very thin (~20 µm)
Gd foil (shown in red) to detect thermal neutrons. To discriminate
thermal neutrons which are emitted from the lunar soil (and are thus
sensitive to the subsurface proton (water) content), the GH sandwich is
shielded from above by a thicker Gd foil which is encased in two thick
Al sheets. The GH sandwich then measures thermal neutrons from below
and the EF sandwich thermal neutrons from above. The lowermost sandwich
is a copy of the IRAS BCsandwich and serves as the final detector in
the stack. The J detector serves as an anticoincidence to discriminate
stopping particles from penetrating ones.

Figure 24: The particle telescope of LND (image credit: LND Team)

Measuring thermal neutrons using Gd converters:
Natural gadolinium (Gd) has a very large cross section for thermal
neutrons (48’890 b). After neutron capture, the Gd nuclei have a
large probability to decay via internal conversion emitting electrons
with energies of 29, 72, 78, and 132 keV (Figure 25).
LND uses a 20 µm thin natural Gd foil as a neutron converter. The
electrons which escape from the foil can then be measured in the
adjacent Si SSDs in anticoincidence with the surrounding detectors.

LND will have
to use a fairly thick (20 µm) foil. While our initial sputtering
tests were successful, the sputtered Gd on the flight detectors showed
bubbles (Figure 26).

Geant4 Simulations: We used Geant4 to simulate the performance of LND. Figure 27
shows the instrument response functions (IRFs) for the inner C segment
(C1) for gamma and neutron particles. It is dominated by neutrons for
energy deposits above 1 MeV.

As discussed in section 3, LND
measures fast neutrons by using an anti-coincidence of C1 with
(B1&B2, D1&D2, and C2). Thus, if only the C1 segment was
triggered, this was due to a neutron or gamma, because both of them
only interact weakly with the Si-nuclei in the detector compared with
charged particles, e.g., protons and electrons. The expected spectrum
of neutral radiation energy deposits in C1 shown in Figure 28
was obtained using a GEANT4-model of the lunar surface neutron and
gamma spectra folded with the LND IRF. It is dominated by gammas at low
energies. Above about 700 keV, neutrons start to dominate, e.g., at
energies above 1 MeV, we expect only about 0.01 gamma, but ~0.03
neutron counts per second.

Data Products: The raw data of LND
is processed by LND on board and telemetered to Earth via an Earth-Moon
L2 relay satellite. LND data products and their cadences are given in
the table to the left. LND provides measurements of interest for
dosimetry, lunar regolith science, as well as heliophysics. After
receipt on ground, instrument response functions will be applied to the
data, and visual inspections will be performed at CAU
(Christian-Albrechts-University in Kiel, Germany) and at NSSC (National
Space Science Center) China. LND data will be made available to the
scientific community via the usual channels.

Number

Measurement

Cadence

1

Dose rate in SI

1 minute

2

Neutral particle dose rate

1 minute

3

Housekeeping

1 minute

4

Pulse height analysis words

1 minute

5

Proton energy spectra up to around 20 MeV

5 minutes

6

Electron spectrum

5 minutes

7

Thermal neutron counts

5 minutes

8

Alpha-particle spectrum up to around 20 MeV /nuc

15 minutes

9

LET-spectra in the range 0.1-430 keV/µm

15 minutes

10

Fast neutrons in the range 1-20 MeV

15 minutes

11

3He spectrum up to roughly 20 MeV/nuc

30 minutes

12

Composition of the radiation

30 minutes

Table 1: Parameters measured

Sensor complement of the Chang'e-4 rover (PCAM, LPR, VNIS, ASAN)

Chang’e-4 mission has been
planned to install six kinds of scientific payloads to complete the
corresponding tasks, three kinds of payloads on the lander are the
Landing Camera (LCAM), the Terrain Camera (TCAM), and the Low Frequency
Spectrometer (LFS), and three kinds of payloads on the rover are the
Panorama Camera (PCAM), the Lunar Penetrating Radar (LPR), and the
Visible and Near-Infrared Imaging Spectrometer (VNIS). The LFS is newly
developed for Chang’e-4 lander, and other five kinds of payloads
are the inherited instruments from Chang’e-3. 44)

Besides the above six payloads,
there are also three international joint collaboration payloads to be
installed on the Chang’e-4 explorer, which are the Lunar Lander
Neutrons and Dosimetry(LND) installed on the lander, the Advanced Small
Analyzer for Neutrals(ASAN) installed on the rover, Netherlands-China
Low-Frequency Explorer (NCLE) installed on the relay satellite.

PCAM (Panoramic Camera)

The objective of PCAM is to obtain
three-dimensional images of the landing and patrolling lunar surface
for investigation of surface morphology and geological structure.

LPR (Lunar Penetrating Radar)

LPR is of Chang'e-3 heritage. The
objective of LPR are the mapping of lunar regolith and detection of
subsurface geologic structures.

VNIS (Visible and Near-Infrared Imaging Spectrometer)

VNIS is of Chang'e-3 heritage. VNIS
is capable of simultaneously in situ acquiring full reflectance spectra
for objects on the lunar surface and performing calibrations. VNIS uses
non-collinear acousto-optic tunable filters and consists of a VIS/NIR
imaging spectrometer (0.45–0.95 µm), a shortwave IR
spectrometer (0.9–2.4 µm), and a calibration unit with
dust-proofing functionality.

ASAN (Advanced Small Analyzer for Neutrals)

ASAN was developed by the Swedish Institute of Space Physics (IRF) in Kiruna.

On April 7, 2018, the Swedish
Institute of Space Physics (IRF) successfully delivered the flight
model of the ASAN (Advanced Small Analyzer for Neutrals) instrument to
the National Space Science Center of the Chinese Academy of Sciences in
Beijing, China. The ASAN instrument will be launched at the end of 2018
onboard the Chinese Chang'e-4 mission to the Moon. 45)

Chang'e-4 relay satellite / Queqiao of China

The Chang'e-4 relay satellite
(CE-4R), named Queqiao ('magpie bridge'), is a precursor to an
unprecedented attempt to soft-land the Chang'e-4 satellite on the lunar
far side in late 2018, when a lander and rover will be send to the
Moon. Since the lunar far side does not face the Earth as the
moon’s orbital period matches its rotational period, a relay
satellite is required to facilitate communications between the
Chang'e-4 lander on the far side of the moon and ground stations on
Earth.

The nickname
Queqiao was announced by CNSA (China National Space Administration) on
25 April 2018, China's Space Day. In a Chinese folktale, magpies form a
bridge with their wings on the seventh night of the seventh month of
the lunar calendar to enable Zhi Nu, the seventh daughter of the
Goddess of Heaven, to cross and meet her beloved husband, separated
from her by the Milky Way. 46)47)

This is the main role of the 425 kg
spacecraft, developed by the China Academy of Space Technology (CAST),
which is being sent into position around six months before the landing
mission in order to test and verify is functions.

The 425 kg relay satellite is based
on the three-axis stabilized CAST-100 minisatellite bus featuring an
130 N hydrazine propulsion system. It carries a deployable 4.2 m dish
antenna for the relay equipment. It provides four 256 kbit/s X-band
links between itself and the lander/rover and one 2 Mbit/s S-band link
towards Earth.

Orbit: Halo orbit of the Earth-Moon
L2 (Lagrangian Point 2), around 65,000 km on the far side of the moon,
so as to be visible to both ground stations on the Earth and the lander
and rover on the lunar far side at all times.

Figure 32: Flight profile of the relay satellite (image credit: CAST)

Secondary payloads:

Two Chinese microsatellites, DSLWP
-A and -B (Discovering the Sky at Longest Wavelengths Pathfinder), also
referred to as Longjiang-1 and Longjiang-2, were launched with the
Chang’e 4 relay mission to conduct astronomical observations from
deep space (Selenocentric, elliptical orbit).The two microsatellites
were developed by the Harbin Institute of Technology. Each
microsatellite has a mass of 47 kg. 49)

They will be inserted into 200 km x
9000 km lunar orbits. The satellites are three-axis stabilized and
carry a radio-astronomy payload featuring two linear polarization
antennas mounted along and normal to the flight direction, which uses
the moon as a shield to avoid radio emanations from earth.
Additionally, the microsatellites carry a KACST (King Abdulaziz City
for Science and Technology) developed micro-optical camera and an
amateur radio communications system.

Figure 33: Photo of the
Longjiang-1 and -2 microsatellites at the launch site, which were
launched to the Moon with the Queqiao/Chang'e-4 lunar relay satellite
on May 20, 2018 (image credit: Harbin Institute of Technology)

The relay satellite orbiting around
the Earth-Moon L2 point is about 60000–80000 km away (halo orbit)
from the lander and the rover working on lunar surface. Under the
constraints of the launching mass and size, the relay communication
link should be optimized in multiple aspects such as the relay
transmission modes and the high gain relay antenna development to
achieve high-bit-rate communication.

Relay communications:
The link between Change’4 relay satellite and lander/rover is
designed to work on X-band. The forward link uses unified carrier-wave
TT&C regime and the backward uses BPSK. The forward relay signal
emitter should be able to scan with complex frequency carrier waves,
similar to ground stations. The relay satellite should be able to
transmit data to Earth and relay with the lander and the rover
simultaneously. To avoid interference, the relay link adopts X-band
channel, and TT&C to Earth chooses S-band unified carrier wave
regime and the data transmission utilizes S-band and BPSK carrier wave
regime. Channel encoding is not used for telecommand transmission and
RS+concatenated convolution channel encoding is adopted for telemetry
and data transmission. Meanwhile for the purpose of synchronous
encoding with the ground, both telemetry and data pseudo-random coded.

Relay communication link design: The link profile among the relay satellite, the lander and the rover and ground stations is as shown in Figure 35.
The forward data is emitted from the relay satellite, the lander and
the rover receive data via omnidirectional antenna and the signals are
modulated in PCM/PSK/PM. The backward data is transmitted via an
omnidirectional antenna, a medium gain antenna or a directional antenna
and is received by the relay satellite. The backward link data of the
rover is transmitted by the omnidirectional antenna or the directional
antenna and is received by the relay satellite. The lander backward
link adopts omnidirectional antenna, medium gain antenna and
directional antenna corresponding to low, medium and high bit rates.
The modulation mode of the backward link is BPSK.

During the powered descent process,
except for setting up X-band omnidirectional forward/backward relay
communication links, the lander and the relay satellite also
communicate via a medium gain antenna to return landing camera data
back to Earth.

While working on the Lunar surface,
the lander and the rover respectively receives forward telecommand
signals via an omnidirectional antenna from the relay satellite. The
relay satellite can send data at two frequency points at the same time
to realize simultaneous control of the lander and the rover. Following
the command, the lander and the rover send backward data (including
telemetry and scientific exploration data) via an omnidirectional
antenna or a directional antenna.

The relay satellite can receive backward data from both the lander and the rover simultaneously.

• August 2019: The first
switch on of the NCLE instrument happened 22 January 2019, where a
first initial sanity check was performed to verify the instrument had
survived the launch, transit to lunar orbit, and 9 months in lunar
orbit (Ref. 59).

- Figure 38
shows the power drawn by the NCLE instrument, which is seen to increase
as progressively more subsystems are switched on. From this figure we
can clearly see that at t=120 seconds the switch-on was commenced, by
powering up the DRS OCXO clock. After that the other subsystems were
switched on one by one. The current drawn by the system is exactly at
the level what it should be, signaling the start of commissioning.

- Currently, the instrument is
undergoing a month worth of observations with antenna’s in stowed
configuration to characterize the noise over an entire lunar orbit.
After this, the antennas will be deployed up to 0.5 meter, after which
they will be extended slowly up to the total 5 meters.

Future Roadmap

- NCLE is not supposed to be the
last step in the quest to observe the early universe. Rather, it is
considered to be a steppingstone mission towards more advanced missions
and instruments. It all starts with the initial NCLE mission as
described in this paper.

- This will be followed by not only
developing the instrument, but a complete platform as well. These
satellites will also be deployed into Lunar orbit to take full
advantage of the lessons learnt during the development of this
instrument.

- The last step is to have multiple
of these satellites not only in lunar orbit, but even further out. The
combination of these instruments at locations which are far apart will
provide an excellent baseline to perform interferometer making all
measurements even more precise.

- Currently, the 2nd phase has just
started with a phase A study. In this study suitable orbits have been
identified and a preliminary system design was derived.

• September 4, 2018: One of
the two microsatellites, launched along with a required communications
relay satellite in May, has quietly been allowing radio operators to
download images from the spacecraft taken along its elliptical lunar
orbit. 50)

- Longjiang-2,
aka DSLWP-B, was developed by students at the Harbin Institute of
Technology (HIT) in the Heilongjiang Province, northeast China. Despite
having a mass of just 47 kg, the tiny satellite managed to use its own
propulsion to slow down and enter lunar orbit while the relay satellite
continued past the Moon to its special destination.

- During its time in orbit
Longjiang-2 has used a student-developed camera to take images of the
Moon, Mars, the Sun and other objects. UHF tests have seen data
transmitted by Longjiang-2 and received and decoded by radio operators
on Earth.

- Unfortunately its partner
microsatellite, Longjaing-1/DSLWP-A, was lost shortly after trans-lunar
injection, and likely remains in distant Earth orbit after a lunar
flyby.

• August 16, 2018: Longjiang-2
(aka DSLWP-B), one of two microsatellites launched along with the
Queqiao relay satellite in May, is still functioning in an elliptical
orbit around the Moon, with amateur radio enthusiasts using the 47 kg
spacecraft for experiments. 51)

- Longjiang-2 carries a camera
developed by KACST of Saudi Arabia, accounting for the final of four
internationally-provided payloads for Chang’e-4, which sent back
these cool images of the Earth and Moon.

- Another onboard imager developed
by students at the Harbin Institute of Technology (HIT) in northeast
China has allowed radio operators to download images. Here is one such
image, of Mare Nubium on the near side of the Moon.

• June
18, 2018: Two microsatellites, Longjiang-1 and Longjiang-2, were sent
into space on 20 May (21:28 UTC) together with the Chang'e-4 lunar
probe's relay satellite from southwest China's Xichang Satellite Launch
Center. 52)

- Longjiang-2 successfully reached
its destination near the Moon on May 25, and entered a lunar orbit with
a perilune at 350 km and an apolune at 13,700 km. However, Longjiang-1
suffered an anomaly and failed to enter lunar orbit, according to CNSA
(China National Space Administration).

- With a mass of 47 kg, Longjiang-2
has become the world's first lunar orbiter developed by a university.
The Longjiang-2 microsatellite carries an optical camera developed by
KACST (King Abdulaziz City for Science and Technology) of Riyadh,Saudi
Arabia, as well as a low-frequency radio detector developed by the
National Space Science Center of CAS (Chinese Academy of Sciences).

- The camera, which began to work on
28 May , has conducted observations of the Moon and acquired a series
of clear lunar images and data, according to Xinhua News.

Figure 41: The released photo
shows part of the Mare Imbrium on the moon. On 14 June 2018, China and
Saudi Arabia jointly unveiled three lunar images acquired through
cooperation on the relay satellite mission for the Chang'e-4 lunar
probe (image credit: CNSA, CLEP, KACST)

• June 15, 2018: China has
provided an update on its Chang'e-4 relay satellite, launched in May in
preparation for a later landing on the far side of the Moon, while also
revealing the status of the Longjiang microsatellites intended for
lunar orbit. A series of stunning images from a Saudi Arabia-developed
camera aboard one of the lunar microsatellites, namely Longjiang-2,
were also released. 53)

Figure 42: A demonstration of
the Lissajous/halo orbit to be used by the Queqiao Chang'e-4 relay
satellite mission (image credit: CASC)

Figure 43: The Earth and Moon
imaged on June 8 by the KACST-developed camera on China's Longjiang-2
microsatellite. The image shows Saudi Arabia on the distant Earth, as
well as the northern hemisphere of the lunar far side, near the
Petropavlovskiy crater (image credit: CNSA/CLEP/KACST)

• June 14, 2018: The relay
satellite for the Chinese Chang'e-4 lunar probe, named Queqiao and
launched on 20 May 2018, entered the Halo orbit around the second
Lagrangian (L2) point of the Earth-Moon system (EML-2), about 65,000
km. from the Moon, at 11:06 a.m. on Thursday, June 14, after a journey
of more than 20 days. 54)

- "The
satellite is the world's first communication satellite operating in
that orbit, and will lay the foundation for the Chang'e-4, which is
expected to become the world's first soft-landing, roving probe on the
far side of the Moon," said Hongtai Zhang, President of the China
Academy of Space Technology (CAST). 55)

- The concept of the Halo orbit
around the EML-2 (Earth-Moon L2 ) point was first put forward by
international space experts in 1950s. While in orbit, the relay
satellite can see both the Earth and the far side of the Moon. The
satellite can stay in the Halo orbit for a long time due to its
relatively low use of fuel, since the Earth's and Moon's gravity
balances the orbital motion of the satellite.

- "From Earth, the orbit looks like
a halo of the Moon, which is where it got its name," said Lihua Zhang,
project manager of the relay satellite. He said the Halo orbit was a
three-dimensional ,irregular curve. It is extremely difficult and
complex to maintain the satellite in orbit. "If there is a tiny
disturbance, such as gravitational disturbance from other planets or
the Sun, the satellite will leave orbit. The orbit period is about 14
days. According to our current plan, we will conduct orbit maintenance
every seven days. It's a new type of orbit, we don't have any
experience. We ran a number of simulations to make sure the design is
feasible and reliable," Zhang added.

- In order to establish a
communication link between Earth and the planned Chang'e-4 lunar probe,
space engineers must keep the satellite stable and control its
altitude, angle and speed with high precision. Next, the team will test
the communication function of the relay satellite.

- With a mass of 400 kg, and with a
design life of three years, the satellite carries several antennas.
One, shaped like an umbrella with a diameter of 4.2 meters, is the
largest communication antenna ever used in deep space exploration, this
according to Lan Chen, deputy chief engineer of the Xi'an Branch of
CAST.

- Tidal forces of the Earth have
slowed the Moon's rotation to the point where the same side always
faces the Earth, a phenomenon called tidal locking. The other face,
most of which is never visible from Earth, is the far side or dark side
of the Moon, not because it's dark, but because most of it remains
unknown. With its special environment and complex geological history,
the far side is a hot spot for scientific and space exploration. The
Aitken Basin of the lunar south pole region on the far side has been
chosen as the landing site for Chang'e-4. The region is believed to
have great research potential.

• June 1, 2018: Queqiao made
its lunar swing-by on May 25, performing a braking burn at 13:32 UTC to
send the communications satellite towards EML-2, some 60-80,000 km
beyond the Moon. 56)

Sensor complement of the Chang'e-4 relay satellite: (NCLE)

On board is a Dutch radio antenna, the NCLE (Netherlands
Chinese Low-Frequency Explorer). The radio antenna is the first
Dutch-made scientific instrument to be sent on a Chinese space mission,
and it will open up a new chapter in radio astronomy. 57)

The NCLE instrument was developed
and built by engineers from ASTRON, the Netherlands Institute for Radio
Astronomy in Dwingeloo, the Radboud Radio Lab of Radboud University in
Nijmegen, and the Delft-based company ISIS — in collaboration
with a team from the Chinese NAOC (National Astronomical Observatory of
the Chinese Academy of Sciences). With the instrument, astronomers want
to measure radio waves originating from the period directly after the
Big Bang, when the first stars and galaxies were formed.

Why is it so important for the
measuring instruments to be placed behind the Moon? Professor of
Astrophysics from Radboud University and ASTRON Heino Falcke: "Radio
astronomers study the universe using radio waves, light coming from
stars and planets, for example, which are not visible with the naked
eye. We can receive almost all celestial radio wave frequencies here on
Earth. We cannot detect radio waves below 30 MHz, however, as these are
blocked by our atmosphere. It is these frequencies in particular that
contain information about the early universe, which is why we want to
measure them."

Special about the radio antenna is
that it will receive low frequency radio waves with a large frequency
range. The instrument passed an important risk assessment review by the
Chinese space agency at the end of April.

"In the past
this was not possible and therefore a receiver with a narrow frequency
band was used, in order to avoid electromagnetic interference of the
satellite itself," explains project leader Albert-Jan Boonstra of
ASTRON. "We have now succeeded in avoiding the electromagnetic
interference and making a broadband receiver. That is, of course, good
news for subsequent missions and can, for example, be used for future
nano-satellites."

In 2016, the NSO (Netherlands Space
Office) and its Chinese counterpart CNSA signed an agreement to
cooperate in this project, which was an elaboration of the Memorandum
of Understanding the two space agencies signed the year before during a
trade mission in presence of the Chinese President Xi Jinping and the
Dutch King Willem Alexander.

A digital controller unit, designed
and supplied by the Somerset West-based SAC (Space Advisory Company),
is part of the NCLE (Netherlands-China Low-Frequency Explorer) as a
science payload on the Chang’e-4 relay satellite which lifted
atop a Long March 4C Rocket from Xichang Satellite Launch Center at
21:28 UTC on Sunday, 21 May 2018. 58)

While the Chang’e-4
satellite’s main mission is to relay messages between Earth and
the Moon, the NCLE instrument will ride along and conduct experiments
into deeper space.

Duncan Stanton, CEO of SAC said
that they are ecstatic to be part of such an unique mission and
especially proud of their engineering team who proved themselves to be
world-class by meeting the ambitious timeline and performance
requirements of the project. They may just have embarked on proudly
flying the South African flag the furthest ever.

Stanton continued that the
controller unit supplied by them forms a critical part of the digital
receiver system for the NCLE instrument. The instrument was built by
the Radboud Radio Lab from the Radboud University, the Netherlands
Institute for Radio Astronomy (ASTRON), and ISIS (Innovative Solutions
In Space) in Delft. The instrument has a primary science objective to
detect low frequency 21 cm hydrogen line emissions from the ‘dark
ages’ period of the universe before stars began to shine.

SAC is a member of the Somerset
West-based SCS Aerospace Group (SCSAG), Africa’s largest
privately-owned group of satellite design and manufacturing companies
with more than 25 years of experience in this domain.

NCLE (Netherlands-China
Low-Frequency Explorer) is a low-frequency payload which is part of the
Chinese Chang’e-4 mission. The NCLE instrument consists of three
5-meter long monopole antennas mounted on the Queqiao satellite to
measure in the 80 kHz -80 MHz radio frequency range. The instrument is
designed to address a multitude of high-profile science cases, but
predominantly NCLE will open up the low-frequency regime for radio
astronomy and will prepare for the ground-breaking observations of the
21-cm line emission from the Dark Ages and the Cosmic Dawn, considered
to be the holy grail of cosmology. 59)

The main goal
of NCLE is to determine the low frequency radio environment at the
Earth Moon L2 point. It serves as a stepping stone mission towards low
frequency observations, aimed at registering the hydrogen line at
different frequencies and thus creating a ‘moving picture’
of the evolution of the early universe.

Dark Ages and Cosmic Dawn:
The signature emission from neutral hydrogen at 21 cm shows a spectrum
over different redshifts that indicates its cosmic abundance as a
function of time. Around the epoch of reionization in the younger
universe, which was when neutral hydrogen was re-ionized by radiation
from early stars, this signature should show a measurable dependence on
redshift as a result in the range from 1 – 80 MHz. The detection
or the constraining of this signature will have a strong impact on
cosmology and science of the early universe, as it provides information
on an epoch in the history of our universe from which we cannot receive
much information through other means.

Solar Bursts:
The Sun, our closest star, affects fundamentally the Earth's ecosystem
and daily life thereby affecting the quality of life on Earth and the
performance of technological systems (e.g. power grid). Superimposed on
the powerful thermal emissions of the quiet Sun are the intense radio
bursts associated with solar flares and coronal mass ejections (CME),
clouds of ionized plasma are ejected into interplanetary space. Despite
their great importance to Space Weather services, the physical
mechanisms governing such events are poorly understood, with the
consequence that neither accurate models nor reliable prediction tools
exist.

The Sun often exhibits magnetic
activity outside of its photosphere, in the form of rapid reordering of
its smaller-scale magnetic field that may be accompanied by mass
ejections. These field reconfigurations generate strong radio emission
through synchrotron radiation from charged particles in these
magnetized regions. The strength of this emission makes this a source
that can be investigated by NCLE relatively easily, establishing
statistics and correlations over a long observing period.

Two main types of radio bursts are
observed from the Sun particularly in its active state: Type II bursts
drive energizing shocks through the solar corona and interplanetary
medium (~1000-2000 km/s), and Type III bursts result from mildly
relativistic (~0.1 - 0.3 c) electron beams propagating through the
corona and interplanetary space that excite plasma waves at the local
plasma frequency but do not create a CME. Type II bursts and their
ensuing CMEs can be followed by NCLE much further out than the few
solar radii possible from the ground because of the ionosphere, see
Figure 46.

Figure 46:
The frequency ranges of various emission mechanisms in the radio band.
The black line shows the local plasma frequency, plotted as a function
of distance from the Solar photosphere. From the Earth’s surface,
observations are limited to the frequency band above ~3 MHz (image
credit: ISIS, Radbaud University, Astron)

Lightning (Earth): Rapid
discharge of built-up electrical charge between different clouds in
Earth’s troposphere or between clouds and the ground generates
bursts of wide-band radio emission. As such, lightning represents one
component of Earth-based RFI with a unique signature that is critical
for us to understand.

Auroral Kilometric Radiation / AKR (Earth):
Charged particles in the Solar wind are deflected by Earth’s
magnetic field. Under certain circumstances, these particles can become
temporarily trapped and emit cyclotron radiation at very low
frequencies. As this phenomenon arises from the interaction of the
Solar wind with the Earth’s magnetic field, measuring AKR allows
us to learn about both of these systems.

Galactic Background:
Interaction of cosmic rays (energetic charged particles) with the
Galactic magnetic field generates a wide-spectrum Synchrotron
background, which is especially prominent in the Galactic plane.
Measurement of this spectral component in the form of a multi-frequency
sky map helps us to understand the geometry of the Galactic magnetic
field and the cosmic ray population in our Galaxy.

Quasi-Thermal Noise (QTN):
Interaction of the instrument antennas with local plasma causes small
voltage variations in them, which manifest as an extra noise component
in the measurements. The measurement of QTN outside of the strong
influence of Earth’s magnetic field enables us to sample local
solar wind properties (possibly allowing us to correlate these with
solar outburst data), as well as to understand its influence on our
other measurements.

Solar System Planetary Emissions:
Analogous to AKR from Earth, the extended magnetospheres of Jupiter and
Saturn can trap charged particles from the Solar wind, which proceed to
generate low-frequency cyclotron radiation. As with Earth, measuring
this AKR helps us to understand the magnetic fields of the most massive
planets in the Solar system, as well as the Solar wind itself.

Bright Radio Transients From Outside The Solar System:
There are many possible processes that might generate low-frequency
radio emission with a rapidly varying temporal signature. One example
is pulsar emission, which tends to have a strong low-frequency
component. Fast Radio Bursts, from hitherto unknown origin, are another
example. An issue with short pulses at low frequencies is that they get
dispersed quite strongly by interstellar propagation. An ability to
measure these transients from space at low frequencies offers the
opportunity to correlate these detections with Earth-based measurements
at higher frequencies. NCLE’s remoteness from Earth and regular
occultation by the Moon, combined with its multidirectional
sensitivity, should offer us ample opportunities to look for these
events.

Engineering challenges

Leaving low Earth Orbit: To
support the science cases, there is no choice but to leave the well
know low earth orbits and go further out. The NCLE instrument managed
to find an agreement with the Chinese CE-4R satellite; a satellite
travelling to the Moon Earth L2 point to act as a relay satellite for a
rover which will land on the far side of the Moon.

Because NLCE is part of a larger
satellite some of the obvious challenges have already been met. The
host spacecraft will provide mechanical, power, communication and
thermal interfaces. However, at the start of the project the host was
already designed and built meaning the NCLE instrument needed to
conform to the design as little changes could be made.

This proved to be just as
challenging as designing without a spacecraft present. The information
to and from the Chinese included large uncertainties as the final
orbit, orientation of the instrument or configuration of the spacecraft
were largely unknown when the NCLE design started. This directly
translates to uncertainties in the design.

Schedule: At the start of
the project only a few things were clear; the system must be below 10
kg, below 50 Watts, will orbit the Earth Moon L2 point and the launch
will be in May 2018.

As the project started in earnest
May 2016, this left only 2 years to develop build and test the
instrument. The whole system architecture tried to follow the CubeSat
approach as much as possible; special space rated components could not
be procured on time, build and test, test and build and keeping the
system as modular as possible to allow for early testing of the
different subsystems. This allowed for all parts to be ready in March
2018, less than 2 years after the project started.

Electromagnetic Interference:
The other big challenge for the instrument is to reduce its
electromagnetic emissions as much as possible. This was needed to
become sky-noise limited and detect the weak signals over the noise the
instrument itself is making.

CubeSats generally are not designed
to be as clean as possible from an EMC point of view. Rather, little
care is given to this part of the design in typical CubeSat missions.
To make matters worse, the interfaces coming from CE-4R also was not
designed to be as clean as possible.

To combat these issues almost the
entire power system had to be redesigned from the ground up. Care was
taken to implement a star grounding scheme such that all power paths
are well defined and controlled. Isolation between the incoming power,
analog power and digital domain needed to be carefully created as well.

Using previous CubeSat designs as a
blueprint, the power system was designed to be as modular as possible
to facility all these requirements. In the end, no less than 4 boards
have been designed, tested and integrated together, a far cry from the
usual one a typical CubeSat needs!

Lastly, the mechanical design had
to contain special measures to make sure internal signals
wouldn’t radiate to one another. This resulted in one dedicated
box for the LNA (Low Noise Amplifier) system and compartments inside of
the EBOX for the final analog stages.

An overview of the system architecture is shown in Figure 49.
The analog parts are indicated in yellow, the digital system in blue,
the antenna in red and the interface electronics in green.

Figure 49:
The NCLE system architecture. Analog parts are yellow, digital system
in blue, antenna in red and the interface electronics in green (image
credit: ISIS, Radbaud University, Astron)

Analog receiver system: The
analog receiver system consists of three antenna elements that pick up
the external RF signals and are connected through RF connections to the
Low-Noise Amplifier and Analog Input Stage electronics. Each antenna is
connected to its own channel of analog electronics, where the RF
signals are conditioned (amplified, filtered, etc.) before being
sampled by the digital receiver system.

In addition to these three
channels, a calibrator is included that produces a calibration pulse
train that is fed into the three LNA input channels providing a signal
reference.

Digital receiver system: The
digital receiver system consists of Analog to Digital Converters where
the analog signals are sampled, a clock module for time
synchronization, an FPGA based programmable logic where signal
processing is performed and mass memory to buffer the data. This
includes for example FFTs, running data reduction algorithms and
storing the data. It is based on CubeSat hardware; an SCS space image
processor board has been repurposed to perform all these tasks.

On-board data handling and interface electronics: The on-board data handling and interface electronics consists of a number of elements.

The power supply unit provides
regulation of the incoming power, galvanic isolation and signal
filtering, and distribution of the different power lines for the analog
and digital electronics. The board is a heavily version of an ISIS EPS
system, with many additions to facilitate the special needs of this
mission.

An ISIS On-Board Computer is
included to provide data handling and to manage the data interfaces
with the Chang’e-4R spacecraft. The control bus interface is a
dual redundant CAN bus, while the payload data transfer is performed
over a dual redundant LVDS bus.

Deployable antenna system:
To provide sufficient gain at low frequencies and the possibility to
provide directionality information, three orthogonal 5-m long antennas
were required. As the antenna system also had to meet strict stiffness,
mass and volume constraints, a new design had to be made.

The antenna element was designed to
be stiff enough to maintain its shape under all possible forces induced
by the spacecraft, such as propulsion firings or attitude maneuvers,
while meeting the mass and volume constraints. A C-shaped carbon fiber
element was developed that could be rolled up on a drum before
deployment.

The antenna deployment mechanism
consists of a motor and gearhead with associated drive electronics,
worm gear, slipring and a drum with the rolled-up antenna element.
Deployment and end-stop switches provide information about the status
of deployment.

Prior to
delivery several tests have been conducted with the NCLE instrument.
Most of these where not conducted with the antennas deployed. If they
were deployed, the environment was not EMC proof. The final tests was
conducted while integrated on the CE-4R spacecraft, showing the
environment most representative of flight conditions so far.

The results of these tests look
promising: in a noisy lab environment the instrument picks up a
significant amount of noise, see Figure 54.
Integrated on the spacecraft, with antenna’s stowed the picture
looks much better, with a noise floor which is much reduced, see Figure
55. The noise floor is found to be around -130 dBm, with the spikes being interference generated by the platform or environment.

Figure 54: The spectrum as seen by the NCLE system in a noisy lab environment (image credit: ISIS, Radbaud University, Astron)

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net).